Root Difference

Greetings,

While I was in the library once, I've seen a neat trick for finding the difference of the roots of a quadratic equation, x2 - x1 that is. Most unfortunately, I have failed to recall it and cannot think of any other way to do it. Except finding the roots and then subtracting, but that is not quite an interesting thing to do, especially, when the equation is rather complex...
I would appreciate some hints.